The beneficial role of erythrocytosis for oxygen (O₂) transport has been questioned by evidence from bloodletting and hemodilution studies as well as by studies that suggested the existence of an optimal hematocrit (Hct) value. To asses to what extent erythrocytosis is beneficial in Andean men at high altitude, we examined and discussed the "optimal" hemoglobin concentration (Hb) using a mathematical approach by modeling the mean venous O₂ pressure (PO₂) and arterial O₂ content (CaO₂) considering for both the relation between Hb and the mean arterial O₂ pressure (PaO₂). Relations of Hb and/or Hct to other physiological variables such as cardiac output (Q) and convective arterial O₂ transport (TO₂) were also discussed revealing the importance of PO₂ in this model. Our theoretical analysis suggests that increasing Hb allows increase and maintenance of PO₂ only with moderate declines in PaO₂ as a consequence of moderate increases in altitude reaching its maximum at the "optimal" Hb of 14.7g/dL. Additionally, Hb corresponding to high CaO₂ and TO₂ values is apparently not quite advantageous for improvement of oxygenation. Furthermore, Chronic Mountain Sickness is discussed as an insightful example of the effects of excessive erythrocytosis at high altitude.